The invention relates to systems and methods for improving the efficiency of a power generating facility that utilizes a turbine.
Turbomachines, such as gas turbines, aero-derivatives, or the like, commonly operate in a combined-cycle and/or cogeneration mode. In combined-cycle operation, a gas turbine is connected to a generator that produces electricity. A heat recovery steam generator (HRSG) receives at least a portion of the exhaust gas produced by the gas turbine, and the HRSG uses the heat energy in the exhaust gas to generate steam. The steam produced by the HRSG flows to a steam turbine connected to another generator which produces additional electricity. In a co-generation operation, a portion of the steam generated by the HRSG is sent to a separate process requiring the steam.
Combined-cycle and cogeneration plants are rated to generate the maximum amount of energy (mechanical, electrical, etc) at minimum fuel consumption while operating at base load. However, if possible, it is desirable to improve the efficiency of the system when operating at base load.
Base load operations, although desired by operators, is not always feasible. There may not be a demand in the energy market (electrical grid, or the like) for all of the energy generated at base load. In these situations, the power plant either must shutdown or operate at part load, where less than the maximum amount of energy is generated. When a turbine in such a power generation facility is operating at part load, the overall efficiency of the system is typically lower than when the turbine is operating at base load. Thus, there is a need to improve the efficiency of the system when operating under part load conditions.
Gas turbines are typically required to maintain emissions compliance while generating power. A gas turbine operating at part load may not maintain emissions compliance if it is running below a particular load point. The “turndown range” is the loading range where the gas turbine maintains emissions compliance. A broad turndown range allows operators to maintain emissions compliance, minimize fuel consumption, and avoid the thermal transients associated with shutting down the power plant under a broad range of electricity output conditions. Thus, it is desirable to make the turndown range as broad as possible.
When it is desirable to make changes to an existing power plant to increase efficiency, any retrofit solutions should preferably minimize modifications to existing equipment. This is desirable both from a cost perspective, and also to avoid negatively impacting plant operability. Further, any retrofit changes should not increase operation or maintenance costs, otherwise the retrofit changes may not be economically worthwhile.
For all the foregoing reasons, there is a need for a system for improving the efficiency of a power generation facility that is operating at either base load or part load conditions. There is also a need for a system for improving the turndown range of gas turbines. However, any changes made to achieve a higher efficiency or to increase turndown range should not significantly impact other plant operations or maintenance requirements. Otherwise the improvements in efficiency and/or turndown range may be outweighed by higher operating or maintenance costs, or reduced efficiency during other operating conditions.